It is generally known that gaseous mixtures, i.e., air and other gas mixtures primarily comprised of at least two components of different molecular size, may be separated by pressure swing adsorption. This is achieved by passing the gas mixture through a column of adsorbent at an elevated pressure ("adsorption pressure") so that one or more of the components is selectively adsorbed. The selectivity is governed by the pore size distribution in the adsorbent and the pore volume of the proper pore size for the particular component. In this process the gas molecules with a kinetic diameter less than or equal to the pore size are retained or adsorbed on the absorbent while larger diameter molecules pass through the column.
While the adsorbent sieves the gas according to molecular size, it is also possible to fractionate the gas mixture by means of the different rates of diffusion of its components into the pore system of the adsorbent. Accordingly, both methods of obtaining enriched or high purity gas by use of pressure swing adsorption will be included within the present discussion and invention.
Typically, pressure swing adsorption systems include at least two columns of adsorbent so that one column may be regenerated while the other is adsorbing. The complementary cycling between regeneration and adsorption is effected when the gas exiting from the adsorption column exceeds the desired composition of the adsorbed components of the gas entering the column. This point is known as the "breakthrough" point. As the breakthrough point is reached, the phase is switched from adsorption to regeneration. Cyclic operation of adsorbent columns permits an almost constant stream of enriched product gas. U.S. Pat. No. 4,376,640; and U.K. No. 2,018,153 are examples of such systems.
To obtain higher than 99.9% purity of enriched gases, it has been disclosed to use more than one adsorption column during the adsorption phase. Up to ten such columns have been proposed, but the increase in purity has been more than offset by the attendant high costs associated with the additional equipment and energy used. A large number of schemes have been proposed to effect separation of various gases including methane, oxygen, nitrogen, argon and the like.
The recovery of nitrogen-enriched gas from air utilizing an adsorption process employing molecular sieve carbons is well known. See for example, Juntgen, et al., U.S. Pat. No. 4,264,339 which describes a two equal sized column adsorption process for the production of nitrogen-enriched gas. This process uses a pressure equalization between the two columns through their tops and bottoms.
Four-column pressure swing adsorption units using molecular sieve carbons have been employed in the separation of oxygen and nitrogen from air. See for example, Vo, U.S. Pat. Nos. 4,376,639 and 4,376,640. In these four-column systems, two columns are arranged in series and act as a single adsorption zone during the high pressure product gas generation or the low pressure regeneration step. The restoration of pressure of one adsorption zone is effected by introducing the gas exiting the second carbon column of a second serially connected two-column adsorption zone into its inlet while feeding the gaseous mixture solely through the inlet of the second carbon column of this second adsorption zone.
A four-column pressure swing adsorption system has been successfully employed in the separation of hydrogen gas from its mixture with carbon dioxide, carbon monoxide, water and methane. See Wagner, U.S. Pat. No. 3,430,418. The columns in this four-column system are arranged in parallel and have identical functions in each cycle of operation.
Typically, the present molecular sieve technology provides a low yield of product gas and requires large amounts of molecular sieve. Additionally, the prior art processes are energy inefficient in their regeneration methods.
It is, therefore, an object of the present invention to obtain an increased yield of enriched gas in a more cost effective and simpler manner. It is a further object of the present invention to provide a method and apparatus for obtaining increased purities of gas in a simple way without the costly equipment associated with the prior art.